Get rid of feature macros

[drahnr]

Max:

Rust "impl specialization" should land Rust 1.9(?) (PR here: rust-lang/rust#30652).

That feature should hopefully allow us to better determine the capabilities of the different frameworks that are compiled in and use more performant operations if possible. Currently we handle this via ugly feature attributes which rely on our knowledge of the implemented operations rather than the type system. The current system of feature flags also requires us to "dumb down" the backends to the capabilities they all have in common in order to retain the backend portability.

[drahnr]

The underlying rustc has been fixed a few years back, so there really is no point to not tackle this.